322 research outputs found
Gentianella canoi S.J.Castillo & J.S.Pringle sp. nov. (Gentianaceae), una nueva especie del PerĂș
A new species of Gentianella (Gentianaceae) is described from the flora of Peru. Gentianella canoi S.J.Castillo & J.S.Pringle sp. nov., which inhabits high Andean rocky slopes of the Department of Ancash. This new species is related to G. gilgiana (Reimers) Fabris ex J.S.Pringle and G. tristicha (Gilg) J.S.Pringle, with which it shares a caespitose habit, numerous very narrow basal leaves, floriferous stems more than 20 cm high, and widely open corollas. It is differentiated from G. gilgiana mainly by its inflorescences, with 5â11 flowers âvs. 2 or 3â, and corollas 21â28 mm long âvs. 45â55 mmâ; from G. tristicha in its opposite leaves âvs. verticillatedâ; and of both species in their pale yellow corollas âvs. red vermillion in G. gilgiana, lilac in G. tristichaâ and in the absence of trichomes in the adaxial surface of the corolla.Se describe una especie nueva de Gentianella (Gentianaceae) para la flora del PerĂș. Gentianella canoi S.J.Castillo & J.S.Pringle sp. nov., que habita laderas rocosas altoandinas del departamento de Ăncash. Esta nueva especie es afĂn a G. gilgiana (Reimers) Fabris ex J.S.Pringle y G. tristicha (Gilg) J.S.Pringle, con las que comparte el hĂĄbito cespitoso, numerosas hojas basales muy angostas, tallos florĂferos de mĂĄs de 20 cm de altura y corolas ampliamente abiertas. Se diferencia de G. gilgiana principalmente por su inflorescencia con 5â11 flores âvs. 2 o 3â y corolas de 21â28 mm de longitud âvs. 45â55 mmâ; de G. tristicha por sus hojas opuestas âvs. verticiladasâ; y de ambas especies por sus corolas de un amarillo pĂĄlido âvs. un color rojo bermellĂłn en G. gilgiana, lilacino en G. tristichaâ y la falta de tricomas en la superficie adaxial de la corola
Low Luminosity States of the Black Hole Candidate GX~339--4. II. Timing Analysis
Here we present timing analysis of a set of eight Rossi X-ray Timing Explorer
(RXTE) observations of the black hole candidate GX 339-4 that were taken during
its hard/low state. On long time scales, the RXTE All Sky Monitor data reveal
evidence of a 240 day periodicity, comparable to timescales expected from
warped, precessing accretion disks. On short timescales all observations save
one show evidence of a persistent f approximately equal to 0.3 Hz QPO. The
broad band (10^{-3}-10^2 Hz) power appears to be dominated by two independent
processes that can be modeled as very broad Lorentzians with Q approximately
less than 1. The coherence function between soft and hard photon variability
shows that if these are truly independent processes, then they are individually
coherent, but they are incoherent with one another. This is evidenced by the
fact that the coherence function between the hard and soft variability is near
unity between 0.005-10 Hz but shows evidence of a dip at f approximately equal
to 1 Hz. This is the region of overlap between the broad Lorentzian fits to the
PSD. Similar to Cyg X-1, the coherence also drops dramatically at frequencies
approximately greater than 10 Hz. Also similar to Cyg X-1, the hard photon
variability is seen to lag the soft photon variability with the lag time
increasing with decreasing Fourier frequency. The magnitude of this time lag
appears to be positively correlated with the flux of GX 339-4. We discuss all
of these observations in light of current theoretical models of both black hole
spectra and temporal variability.Comment: To Appear in the AStrophysical Journa
Silicon isotopes reveal recycled altered oceanic crust in the mantle sources of ocean island basalts
EP thanks the Chateaubriand STEM fellowship program for funding. FM thanks the European Research Council under the European Communityâs H2020 framework program/ERC grant agreement #637503 (Pristine) and the Agence Nationale de la Recherche for a chaire dâExcellence Sorbonne Paris CitĂ© (IDEX13C445) and for the UnivEarthS Labex program (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02). PS thanks the support of the Marie Curie FP7-IOF fellowship âIsovolcâ.The study of silicon (Si) isotopes in Ocean Island Basalts (OIB) has the potential to discern between different models for the origins of geochemical heterogeneities in the mantle. Relatively large (âŒseveral per mil per atomic mass unit) Si isotope fractionation occurs in low-temperature environments during biochemical and geochemical precipitation of dissolved Si, where the precipitate is preferentially enriched in the lighter isotopes relative to the dissolved Si. In contrast, only a limited range (âŒtenths of a per mil) of Si isotope fractionation has been observed from high-temperature igneous processes. Therefore, Si isotopes may be useful as tracers for the presence of crustal material within OIB mantle source regions that experienced relatively low-temperature surface processes in a manner similar to other stable isotope systems, such as oxygen. Characterizing the isotopic composition of the mantle is also of central importance to the use of the Si isotope system as a basis for comparisons with other planetary bodies (e.g., Moon, Mars, asteroids). Here we present the first comprehensive suite of high-precision Si isotope data obtained by MC-ICP-MS for a diverse suite of OIB. Samples originate from ocean islands in the Pacific, Atlantic, and Indian Ocean basins and include representative end-members for the EM-1, EM-2, and HIMU mantle components. On average, ÎŽ30Si values for OIB (â0.32 ± 0.09â°, 2 sd) are in general agreement with previous estimates for the ÎŽ30Si value of Bulk Silicate Earth (â0.29 ± 0.07â°, 2 sd; Savage et al., 2014). Nonetheless, some small systematic variations are present; specifically, most HIMU-type (Mangaia; Cape Verde; La Palma, Canary Islands) and Iceland OIB are enriched in the lighter isotopes of Si (ÎŽ30Si values lower than MORB), consistent with recycled altered oceanic crust and lithospheric mantle in their mantle sources.PostprintPeer reviewe
Eclipsing Broad Emission Lines in Hercules X-1: Evidence for a Disk Wind?
We present disk wind model calculations for the broad emission lines seen in
the ultraviolet spectra of the X-ray binary Hercules X-1. Recent HST/STIS
observations of these lines suggest that they are kinematically linked to the
orbital motion of the neutron star and exhibit a red-shifted to blue-shifted
evolution of the line shape during the progression of the eclipse from ingress
to egress which is indicative of disk emission. Furthermore, these lines are
single-peaked which implies that they may be formed in a disk wind similar to
those we have proposed as producing the broad emission lines seen in the UV
spectra of active galactic nuclei. We compute line profiles as a function of
eclipse phase and compare them to the observed line profiles. Various effects
may modify the appearance of the lines including resonant scattering in the
wind itself, self-shadowing of the warped disk from the central continuum, and
self-obscuration of parts of the disk along the observer's line-of-sight. These
latter two effects can cause orbital and precessional phase dependent
variations in the emission lines. Hence, examination of the line profiles as a
function of these phases can, in principle, provide additional information on
the characteristics of the disk warp.Comment: 32 pages, 9 figures, LaTeX, Accepted for publication in Ap
Study of Structural and Magnetic Properties of Iron-Rich Mixed Rare-Earth NdDyFe (17-y-x)CoxSiy Compounds
A series of NdDyFe(17-y-x)CoxSiy solid solutions with = 2 and 3 and = 0.5 1.0 and 1.5 were prepared by induction melting stoichiometric amounts of high-purity elements. The postannealed samples consist of two phases belonging to the space groups R3 m and P63 mmc . The lattice parameters and the unit cell volumes were calculated from the refinements of the magnetic and structural unit cells using the FULLPROF version of the Rietveld program. For a fixed content of Co, the maximum Curie temperatures (305 C to 405 C) were observed in samples with = 1 and having two phases, a disordered rhombohedral (DR) structure and a disordered hexagonal (DH) structure. An increase in the Curie temperature of 70 C per atom of cobalt is observed in NdDyFe(17-y-x)CoxSiy with y = 1 and x\u3c 3, suggesting that with a suitable choice of rare earths this DR phase may be a promising candidate for high-energy product permanent magnets. The magnetization versus temperature (M versus T) plots of the solid solutions, which consist of two phases, exhibit only a single magnetic ordering transition temperature
X-ray Investigation of the Critical Exponent η in Argon
Measurements were made of the small-angle x-ray scattering intensity from argon near the critical point. After the scattering curves are corrected for all known effects except those resulting from irrelevant variables, a value of the critical exponent η=0.09±0.02 is obtained, while if a correction for the estimated effect of irrelevant variables is also made, η=0.03±0.03. The scattering data for argon therefore show that if irrelevant variables can be neglected, the critical exponent η for argon is in good agreement with the value obtained by Warkulwiz, Mozer, and Green from small-angle neutron scattering data for neon. These values of η, however, are clearly greater than those obtained by calculations using high-temperature expansions and renormalizationgroup techniques. On the other hand, if the effects of irrelevant variables are given by the estimate used in correcting the scattering data, the η value computed from the scattering curves agrees with the theoretical results
On the angular momentum evolution of merged white dwarfs
We study the angular momentum evolution of binaries containing two white
dwarfs which merge and become cool helium-rich supergiants. Our object is to
compare predicted rotation velocities with observations of highly evolved stars
believed to have formed from such a merger, including RCrB and extreme helium
stars. The principal study involves a binary containing a 0.6 solar mass CO
white dwarf, and a 0.3 solar mass He white dwarf. The initial condition for the
angular momentum distribution is defined where the secondary fills its Roche
Lobe. We assume conservation of angular momentum to compute the angular
momentum distribution in a collisionless disk and subsequently in the giant
envelope. At the end of shell-helium burning, the giant contracts to form a
white dwarf. We derive the surface rotation velocity during this contraction.
The calculation is repeated for a range of initial mass ratios, and also for
the case of mergers between two helium white dwarfs; the latter will contract
to the helium main-sequence rather than the white dwarf sequence. Assuming
complete conservation of angular momentum, we predict acceptable angular
rotation rates for cool giants and during the initial subsequent contraction.
However such stars will only survive spin-up to reach the white dwarf sequence
(CO+He merger) if the initial mass ratio is close to unity. He+He merger
products must lose angular momentum in order to reach the helium main sequence.
Minimum observed rotation velocities in extreme helium stars are lower than our
predictions by at least one half, indicating that CO+He mergers must lose at
least one half of their angular momentum.Comment: 11 pages, 11 figures, MNRAS in pres
Numerical modelling of the rise of Taylor bubbles through a change in pipe diameter
The rise of Taylor bubbles through expansions in vertical pipes is modelled using Computational Fluid Dynamics. The predictions from the models are compared against existing experimental work and show good agreement, both quantitatively and qualitatively. Many workers, including the present work, find that, as the bubble passes through the expansion, it will either remain intact or split into one or more daughter bubbles. We find that the critical length of bubble, defined as the maximum length that will pass through intact, is proportional to the cosecant of the angle of the expansion. Further, we show that for an abrupt expansion, the critical bubble length became unaffected by the walls of the upper pipe as the diameter was increased
The ``Outside-In'' Outburst of HT Cassiopeiae
We present results from photometric observations of the dwarf nova system HT
Cas during the eruption of November 1995. The data include the first
two--colour observations of an eclipse on the rise to outburst. They show that
during the rise to outburst the disc deviates significantly from steady state
models, but the inclusion of an inner-disc truncation radius of about 4
and a ``flared'' disc of semi-opening angle of produces
acceptable fits. The disc is found to have expanded at the start of the
outburst to about , as compared to quiescent measurements. The
accretion disc then gradually decreases in radius reaching during
the last stages of the eruption. Quiescent eclipses were also observed prior to
and after the eruption and a revised ephemeris is calculated.Comment: 9 pages, 11 figures, to appear in MNRA
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